The advantages of working a metal in its solid state, the equilibrium state under most working conditions, rather than its liquid state are well known. The enhanced reactivity of the metal in the liquid state makes it prone to reaction with the atmosphere or mold, die or furnace elements, resulting in the formation of solid inclusions and/or incorporation of dissolved gases into the melt. Processes involving molten metal also necessarily involve phase transformation associated with solidification shrinkage, evolution of dissolved gases and a number of casting defects.
On the other hand, working metal in the solid state requires a large amount of energy to deform the metal, necessitating heavy and expensive machinery. It is known to extrude a material, typically a soft metal (e.g., aluminum, copper, magnesium, zinc, silver and alloys thereof in the form of a continuous cable, tube or ribbon through a die by maintaining frictional engagement of the material with a passageway defined by driving and non-driving surfaces, such that frictional drag maintains extrusion pressure and urges the material through the die ("frictional extrusion"). This process has been typically used for preparing continuous lengths of cable or tubing. The reader is directed to the prior art on continuous extrusion for more specific details, e.g., GB 1,370,894, GB 1,566,152, and GB 1,590,766.
It is desirable to develop a process capable of use for preparing massive structures of non-uniform cross-section because the process is relatively inexpensive in comparison to conventional metal-working processes, such as forging, and provides inherently higher quality materials than some less expensive casting processes. However, extrusion of large articles with non-uniform cross-sectional areas results in variation of extrusion processing conditions, such as velocity and pressure, along the extrusion pathway. Such processing variations can result in increased porosity and/or inclusions, as well as other structural defects in the final product.
In conventional extrusion processes, the surface over which extrusion occurs is small and the extrusion pressure is correspondingly small, as well. When it is desired to extrude a metal into a die chamber of reasonable complexity, the material must move (be extruded) over a large regions of varying cross-sectional area. The forces on the material are required to be very large. Hence, conventional continuous extrusion processes are not readily amenable to the preparation of large metal pieces.
In U.S. Pat. No. 5,383,347, the inventors disclosed a method of using frictional extrusion to continuously form a shaped article. The method and apparatus direct a frictionally extruded feed material into a holding chamber and, from there, into one of a plurality of die chambers. The method and apparatus permit the extrusion of large metal pieces of complex shape that can not be readily prepared by conventional extrusion processes. The method is limited, however, in that extrusion pressures no greater than that of the frictional extrusion source can be exerted on the feed material. Additionally, residence time in the extrusion process can be long and frictional losses to the chambers and conduits result in a further reduction in extrusion pressure. Thus, some porosity may remain in the final piece, which may be unacceptable for structural or load bearing articles.
There have been attempts in the prior art to provide a feed material that is in the semi-solid or plastic state for use in die casting operations. These techniques have been commercially unsuccessful, largely because of the high expense associated with the preparation of the semi-solid starting material. The starting material for semi-solid metal casting is a continuously cast fine-grain billet produced using electromagnetic stirring to produce a grain texture of solid spheroids suspended in molten metal. The billets are then inductively heated with very tight temperature tolerances and are automatically loaded into the sleeve of a die casting machine. The material is cast when it is about 60% solid and 40% liquid. The material is stiff enough to retain its shape, yet the globular microstructure of the solid spheroids suspended in molten metal allows the material to be cut like butter. Although the semi-solid metal provides good properties for casting, materials processing cost makes this method cost-prohibitive.
It would be desirable to have an apparatus and method which would allow the forming of a plastic material into complex shapes with reduced porosity and gas entrapment in a cost-effective process.
It is an object of the present invention to provide a method and apparatus for the forming of complexly shaped metal articles with reduced porosity and increased strength.
It is an object of the present invention to provide a method and apparatus for the extrusion of large metal pieces with complex shape that cannot be readily prepared using conventional extrusion processes.
It is a further object of the present invention to provide a method and apparatus for the formation of a plastic material into a complex shape which provides additional forming pressure to reduce porosity and gas entrapment.
It is a further object of the present invention to provide a method and apparatus for forming complexly shaped structural and load-bearing articles.
It is a further object of the present invention to provide a method and apparatus for forming complex shaped structural articles which can be subsequently heat treated with retention of excellent surface finish, that cannot be readily prepared using high pressure die casting techniques.
It is yet a further object of the invention to provide a method and apparatus for forming low-porosity and low-inclusion content articles that require minimum machining and finishing to achieve final dimensions and surface finish, that cannot be readily prepared using casting processes.
The present invention provides a high quality article with excellent structural and dimensional properties, at a lower cost than conventional metal-working processes.